Understanding and controlling optical damage in high energy laser systems remains a technologically challenging and scientifically complex problem. Over the years, various methods have been used to reduce or mitigate laser damage in optical components using high average power ultraviolet laser. In some conventional processes, mid- or far-infrared (IR) laser-based localized treatment of surface damage on fused silica lenses has been used to reduce or eliminate the observed shot-to-shot damage growth while producing optically-benign local morphologies with minimal residual stress. A particular method showed that the damage threshold of CO2 laser treated silica can be greatly enhanced, presumably through annihilation of light absorbing defects, capillary-driven crack healing and toughening of the glass. In addition it has been observed that a sufficiently long ramp down in laser power will reduce any thermally-induced residual stresses to acceptable levels. However in most of the conventional damage mitigation techniques, a non-planar morphology associated with both the missing material of the original damage site and any additional material loss from laser processing necessarily leads to modulation of subsequent propagating UV light and the possibility of damaging other optics in the system.
In addition to minimal light modulation, satisfactory mitigation of optical damage requires low residual stresses, which can be accidentally introduced through heating, and maintaining at least as high a damage threshold as the original optical surface. Various laser-based methods which flow and remove damage material through evaporation have been used to arrest the damage and yield high damage threshold surfaces, but generally the residual stresses and morphology were far from ideal. Most recently, ‘rapid evaporation’ approaches (a.k.a. Rapid Ablation Mitigation, RAM), which ‘machine’ away damaged material using tightly-focused (˜130 um) and high irradiance (>1 kW/cm2) IR light may be used for both input and exit surface damage sites on wedge focus lenses. These methods limit the morphology to near cone shapes, devoid of rounded rim structures which tend to focus light downstream and result in ineffective mitigation. Furthermore, since the exposures are short in duration, a limited amount of heat is deposited and therefore residual stress is of less concern. In terms of up-scaling, however, increases in evaporated (a subsequent nanoparticle) material contamination can be a concern; both in terms of UV light absorption and damage initiation. Moreover, light intensification and scalability is still a challenge for laser machining methods, particularly when considering the interaction of mitigation sites with each other, other surface features (e.g., scratches) or laser beam modulation through interference of propagated UV light. Even in an ideal case of infinitely steep walls, i.e., an ideal obscuring aperture, it is well known that peak-to-mean intensifications can still result as high as 1:1.4 due to Fresnel diffraction. Specifically, the steep angles required to refract and defocus light incident on input mitigation sites places particular strain on the capability of current RAM protocols.
Minimally-evaporative approaches (T<2300 K) suggested by some conventional methods may be effective for limited-size single sites (<100 μm) and scratches but are difficult to scale along these dimensions due to thermocapillary effects which tend to scale with mitigated area and produce significant amounts of light intensification. As noted above, alternative ‘rapid evaporation’ approaches (T>3500 K), which precisely ‘machine’ away damaged material using tightly-focused (˜130 μm) and high irradiance (>1 kW/cm2) IR light have also been considered for larger (100˜500 μm diameter) damage sites. Excessive removal of material also leads to a highly modified optic which may make the optic unsuitable for its intended operation. In either evaporative or non-evaporative approaches, however, the resulting damage mitigation is necessarily limited to a finite incident light intensity due to the deviation from an ideal flat surface.
There is a need in the art for techniques to replace material lost in the original damage event with high damage threshold material and thus restore the integrity of the wave propagating media/optical component, e.g., by in-filling the lost material and restoring a planar surface.